1. Field of the Invention
The present invention relates to an application of phase conjugate optics to optical communication and, more particularly, to an optical communication method and an optical communication system that use optical phase conjugation.
2. Description of the Related Art
Use of nonlinear optics provides new capabilities and improved characteristics not attainable by the conventional optical technologies. Especially, use of a phase conjugate light permits to compensate a phase fluctuation and a chromatic dispersion in a transmission path. A method of compensating a fiber chromatic dispersion and a method of compensating a high-speed optical pulse distortion by a nonlinear optical effect, both methods being based on the above-mentioned phase-conjugate light, are disclosed in Japanese Patent Application No. 5-221856. The present application is an extension of the above-mentioned patent application and applies optical phase conjugation to optical frequency division multiplexing (OFDM).
Conventional optical communication systems are constructed mainly by use of optical components having linear optical characteristics; therefore, the conventional systems are simple in construction but limited characteristics and functions. Especially, these days, a non-repeating system or an optical amplifier repeating system extending as long as several hundred to several thousand kilometers is being implemented. And a signal speed of such a system extends as high as several Gb/s to several tens of Gb/s. Further, to significantly increase a processing capacity of such a system, many researches are being made about higher-speed time division multiplexing (TDM) and OFDM in the electrical and optical processing stages and transmission methods based on these techniques. These systems have many problems to be solved. One of the most significant problems is the effect of fiber chromatic dispersion or group velocity dispersion (GVD). The GVD to be caused in an optical transmission path degrades transmission characteristics, which in turn limits transmission distances.
One of the conventional actions against the GVD is to minimize the dispersion of the optical fiber. For examples, already realized is an optical fiber in which the dispersion is substantially zero in 1.3 .mu.m band and 1.5 .mu.m band, which are practical transmission wavelength bands. Also, a research is being made into a system in which, to minimize a chirping (dynamic wavelength fluctuation), a laser diode is not modulated directly but a light emitted from the regularly-driven laser diode is externally modulated with an optical modulator. For an optical modulator having an excellent modulation characteristic, a LiNbO.sub.3 Mach-Zehnder type optical modulator has been developed. Further, researches are being made into a method in which a signal light to be transmitted is provided with a chirping in advance to made compensation by the GVD in the transmission path and into a method in which the receiver side makes the dispersion compensation optically or electrically. Meanwhile, the possibility of applying phase conjugate optics to optical communication is described in "Compensation for channel dispersion by nonlinear optical phase conjugation", A. Yariv, D. Fekete, and D. M. Pepper, Opt. Lett., vol. 4, pp. 52-54, 1979.
As with the case of single-channel transmission, following factors limit the transmission speed and transmission distance in OFDM:
(a) the waveform distortion caused by the multiplied effect of GVD and optical Kerr effect; PA1 (b) the influence of channel-to-channel crosstalk.
The crosstalk of (b) is mainly caused by the four-wave mixing (FWM) inside the optical fiber, the FWM occurrence efficiency being largely dependent on light intensity and phase matching. To be specific, use of a dispersion-shifted fiber (DSF) having a small GVD in order to avoid the problem of (a) above provides a condition in which it is easy to attain the FWM phase matching, thereby limiting a signal power of each channel, a channel-to-channel interval, and a transmittable distance. Namely, the transmission of a limited multiplexing density is performed with a limited S/N ratio.
On the other hand, use of a fiber having a relatively large dispersion value in order to suppress the FWM causes the problem of (a) to limit the signal speed of transmission and the transmittable distance. In either case, optical frequency division multiplexing (OFDM) can be implemented only in a fairly limited condition. It should be noted that the problem of (a) is solvable by applying phase conjugate optics as disclosed in the above-mentioned Japanese Patent Application No. 5-221856.